As this type of power converter, a pulse-width modulation (PWM) inverter that turns on or off multiple semiconductor switching elements by employing pulse-width modulation is known. The PWM inverter has a semiconductor switching element included in each of upper and lower arms of each of phase transformation circuits. The semiconductor switching elements included in the upper and lower arms respectively are complementarily turned on or off with a pulse-width modulated switching signal, whereby an alternating-current (ac) output voltage having the amplitude and frequency thereof controlled is generated. The pulse-width modulated switching signal is produced by comparing a voltage command signal with a carrier wave using a PWM pattern arithmetic unit. As the carrier wave, a triangular wave is often adopted.
For pulse-width modulation, an asynchronous pulse-width modulation method is often employed. In the asynchronous pulse-width modulation method, the frequency fc of a carrier wave is retained at a certain value irrespective of the conversional fundamental frequency f of a PWM inverter, that is, the fundamental wave frequency of the ac output voltage of the PWM inverter. The asynchronous pulse-width modulation method is often adopted by reason that: the number of times of switching of a semiconductor switching element per a unit time can be specified; and the updating timing for a voltage command is synchronous with the peak of the carrier wave, and can be readily controlled using a microcomputer. A microcomputer including an asynchronous pulse-width modulation circuit may be employed.
In the asynchronous pulse-width modulation method, the carrier frequency fc of a carrier wave has to be determined to be a sufficiently high frequency with respect to the fundamental frequency f of the PWM inverter. For example, a page 44 of a literature titled “Theory of AC Servo System and Practice in Designing” (Vol. 4, 1997) published from Sogo Densi Shuppan Inc. reads that fc/f has to be 9 or more. However, depending on a field to which the PWM inverter is applied, a large-capacity semiconductor switching element is adopted as the semiconductor switching element. Therefore, the carrier frequency fc may not be set to a sufficiently high value with respect to the fundamental frequency of the PWM inverter. When fc/f cannot be made sufficiently high, such adverse effects arise that: a vibration called a beat occurs in the ac output voltage of the PWM inverter so as to markedly degrade the precision in the ac output voltage of the PWM inverter; and pulsation occurs in a load current. The beat is a signal having at least one frequency expressed as fc−n×f (where n denotes positive and negative integers), or a combination of signals having the frequencies.
For suppression of the beat, adoption of, for example, a synchronous pulse-width modulation method has been proposed in the past. The synchronous pulse-width modulation method is intended to upgrade the precision in the ac output voltage of the PWM inverter by setting the carrier frequency fc to a frequency that is an integral multiple of the fundamental frequency f of the PWM inverter according to a change in the fundamental frequency f, or by synchronizing the switching operations of the semiconductor switching elements, which are included in the upper and lower arms of each of the phase transformation circuits of the PWM inverter, with the fundamental frequency of the PWM inverter. As long as the synchronous pulse-width modulation method is adopted, since the waveform of the ac output voltage of the PWM inverter is synchronous with a voltage command waveform, the beat can be suppressed. However, in the asynchronous pulse-width modulation method, if the synchronous pulse-width modulation is also adopted, addition of a special device or remodeling is needed. This poses a program in that the cost increases.
In JP9-238472A (patent document 1), a proposal is made of a method in which: the pulse width for a switching pattern relevant to a semiconductor switching element of a PWM inverter is integrated in order to estimate the ac output voltage of the PWM inverter; and a difference of the estimated ac output voltage from a voltage command is added to a voltage command of the next time in order to perform compensation for suppression of a beat. Moreover, in JP2005-224093A (patent document 2), a proposal is made of a method of correcting the frequency of a carrier wave so as to suppress the beat.
Patent document 1: JP9-238472A
Patent document 2: JP2005-224093A